1. Field of the Invention
The present invention relates to a plain bearing or bushing having both a ramped and a cylindrical surface profile for distributing lubricating fluid to load-bearing surfaces under both hydrodynamic wedge film lubrication and conventional cylindrical film lubrication.
2. Description of Prior Developments
Rotational motion of a shaft is commonly used to maintain a load supporting lubricant film between annular support surfaces provided on the inner surfaces of bearings and bushings, (hereinafter collectively referred to as bushings) and the outer surface of the shaft. Rotation of the cylindrical shaft draws lubricant into one or more small clearance spaces between the shaft and the load bearing surfaces of the bushing.
The internal support surfaces of the bushing may include a fluid entrance surface spaced radially from the shaft side surface, a land surface having a minimal clearance with respect to the shaft surface and a concave ramp surface joining the fluid entrance surface and the land surface. Lubricating oil is drawn in a circumferential direction from the fluid entrance surface along the ramp surface and onto the land surface.
The oil is wedged into a small radial clearance between the land surface and the shaft side surface to form a pressurized load supporting film. The pressurized wedge-shaped oil film is not readily displaced out of the small clearance space, and is thus able to absorb or sustain relatively large radial shaft loads. This type of bearing lubrication is sometimes referred to as hydrodynamic wedge film lubrication.
Although hydrodynamic wedge film lubrication provides satisfactory results when used in many conventional bearing and bushing applications, the presence of ramped bearing surfaces over the full axial extent of the bearing or bushing decreases the stability provided to a rotating shaft as compared to a nonramped or nominally cylindrical bearing support surface. Moreover, such a ramped design produces a larger leak path for lubricant to escape from between the bearing and shaft.
U.S. Pat. No. 1,236,511 shows a bearing construction that utilizes wedge film lubrication of the above-mentioned type wherein oil is introduced to the bearing through four axially extending grooves in the bearing inner surface. As the shaft rotates, the oil clinging to the shaft surface is drawn circumferentially into small clearance spaces located midway between the grooves. The oil film established in the small clearance spaces provides a low friction support for the shaft, thereby protecting the shaft and bearing against contact and wear.
The circumferentially moving oil film is confined to circumferential motion by rims or lands that form shoulders along edge areas of the wedge film surfaces. A disadvantage of such rims is that if they are fully effective they can be in direct contact with the shaft surface, thereby producing frictional wear. Also, by confining the oil to a circumferential motion, any circulation of oil through the bearing is prevented because there is no convenient oil path out of the bearing. The oil will endlessly circulate in a circumferential direction around the bearing, thereby eventually heating the oil and generating carbon particulates.
Other patents showing wedge film lubrication achieved by circumferential oil motion are U.S. Pat. Nos. 2,631,905 and 3,680,932. In these patented arrangements, the edge areas of the wedge film surfaces are bounded by endless circular rim or land surfaces designed to prevent axial leakage of the circumferentially moving oil film away from the wedge film surfaces.